I loved it. The teachers let me teach an art and drawing class in 2nd grade. Another student taught an algebra class (he's a brilliant cancer researcher now). I co-wrote, illustrated, and sold a comic book with another classmate in 4th grade during class time.
It allowed me to complete the curriculum on my own time and held me accountable.
Montessori was something I made my own, not something that was happening to me.
The transition out wasn't that bad. I did 5th grade again (June baby) at a standard private school. The biggest things: - I had never written a formal essay, or an essay outline. - I had never taken nor prepared for a real test
Like others here, I never really enjoyed school as much after Montessori.
For the right kid, it can cultivate curiosity, independence, collaboration, and a love for self managed learning. For the wrong kid, it can be an unstructured nightmare.